Christoph,

The PAG in the SU-8 resin is actually very stable within the material, and
undergoes virtually no diffusion into the unexposed regions.

There are a couple of possibilities as I see it.  First is as you said, it may
have to do with the reflections from the Pyrex (or even from the wafer chuck
underneath).  Pyrex has two reflecting surfaces, rather than just one.  If the
chuck is considered as well, you will also have a non-mirror polished surface
(typically no better than 1um roughness) for reflection.

A second possibility may be the instance of low-wavelength (<350nm) radiation in
the exposure source.  SU-8 is highly sensitive to this band of exposure, so even
a small amount coming through can have a pretty profound effect, especially
during an overexposure.

Best Regards,
Chad Brubaker

-----Original Message-----
From: Christoph Friese
Sent: Thursday, September 29, 2005 3:54 AM
To: [email protected]
Subject: [mems-talk] SU-8 over-exposure

Hello,

I'm working on SU-8 2010 for layers with a thickness of about 20µm on Pyrex.

I exposed these layers with 120sec, 60sec and 30sec (with 9mW/cm2) and I'm
observing some kind of overexposure (extrem with 120sec and nearly invisible

at 30sec).

[Pictures at http://www.imtek.de/micro-optics/SU8/SU8_2010_Exp.jpg]

All three wafers are treated the same way, except the exposure dose. Could
someone tell me what is exactly happening in the areas between the tines?

Is this kind of a diffusion of the activated acid or just internal
reflections
between the substrate (Pyrex) and the mask?